The Mac editions of Valve's celebrated shooters Half-Life 2, Half-Life 2: Episode One and Half-Life 2: Episode Two are now available through the digital distribution platform, while Tyrannis--the lastest update to CCP's space-faring MMO EVE Online--will hit "later today" and marks "Steam's first day-and-date release for the Mac and PC."
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Those that already own any of the Half-Life 2 titles via Steam PC will be able to download the respective Mac edition for free, thanks to a feature called SteamPlay that enables "customers to purchase a game once and play it on all Steam supported platforms."

In addition, all three Half-Life 2 titles support Steam Cloud--"allowing players to save their game progress on one computer, then seamlessly resume from that save point on another computer (Mac or PC)"--while Half Life 2 and Half-Life 2: Episode One have been updated on both PC and Mac to include new Steam Achievements.